1. Field of the Invention
The invention relates to a semiconductor integrated circuit having an output circuit.
2. Description of the Related Art
In general, a semiconductor integrated circuit has an internal circuit and an output circuit, and various control signals generated by the internal circuit are supplied as output signals to an external device through the output circuit. The output circuit is formed of a switching circuit. The output signal is thus a signal alternating between a high level and a low level, and its transition portion between the low level and the high level forms a step pulse and contains various frequency components (harmonic components) by the Laplace transformation or Fourier transformation. Since such frequency components of the step pulse induce electromagnetic radiation noise by its relation with the output impedance of the output circuit, the circuit is designed so as to provide the output impedance with a suitable value.
A switching regulator circuit in which electromagnetic radiation noise due to a switching noise is reduced is described in Japanese Patent Application Publication No. 2003-153526.
However, the built-in control of the output impedance of the output circuit described above makes the rise and fall times of the output signal constant. In other words, the rise and fall slopes of the output signal are made constant. Then, the step pulse of the output signal still has specific frequency components after all, and this causes electromagnetic radiation noise (power supply noise, signal radiation noise).
On the other hand, a reference clock with a small frequency drift is needed for an application device such as a tuner, an imaging system or the like. Such a reference clock is likely to be influenced by electromagnetic radiation noise associated with the output signal (the step pulse) of the output circuit, causing a problem for the application.